Various medical patients lose voluntary control over their bladder and/or bowel. Although vesicostomy, augmentation cystoplasty or an artificial sphincter implanted around the urethra are commonly used to provide partial control over the evacuation function of the bladder and to control continence, these solutions have drawbacks well known to those skilled in the medical profession and related arts. Other patients who achieve a modicum of control over their bladder functions are equally in need of a system to rehabilitate their nerve and muscle dysfunctions. Similar problems arise in respect to involuntary bowel control.
The physiology of the bladder and bowel is closely linked to the urethral muscle physiology of the pelvic floor (levator and muscle) and its related urethral and anal sphincters. For the bladder to store urine and for the bowel to serve as a reservoir for feces, two opposite, but complementary, behaviors are found. In particular, for storage, the bladder and rectum must relax and the urethral and anal sphincters must remain contracted. The reverse is true during evacuation of either urine or feces, i.e., the urethral or anal sphincter relaxes, along with the pelvic floor, and subsequently the bladder and rectum contracts.
The sequence will reverse once voiding and defecation is completed, i.e., the sphincters and pelvic floor muscles will revert to their tonic closure states and the bladder and rectum will revert to their storage states. This behavior has been demonstrated by simultaneous manometric (or EMG/pressure) recordings of this bladder/rectum, urethral/anal behavior during filling and emptying of the bladder. This sequence of events is well-established and is accepted universally.
Reduced bladder capacity and lack of volitional urinary voiding are experienced by spinal cord injured patients. With the present state-of-the-art, implanted pulse generators that are connected to electrodes attached to sacral roots to electrically stimulate the sacral roots provide patient-controlled bladder voiding. Additionally, dorsal rhizotomies are used to improve the bladder capacity, which is reduced by hyperactivity of the afferent fibers in the dorsal roots.
Currently, voiding by electrical stimulation of the sacral roots is accomplished by stimulating both the somatic and the parasympathetic nerves in the sacral roots. This technique causes both the striated sphincter muscles and the detrusor smooth muscles to contract simultaneously. As a result, the increased sphincter pressure is still able to block the passage of urine in spite of the increased bladder pressure. After a few seconds of stimulation, the electrical stimulus pulses are turned off and the striated somatic muscles relax to decrease the sphincter pressure before the slower smooth muscle of the bladder relaxes, thus providing a pressure differential, higher in the bladder, so that a momentary passage of urine results. The electrical stimulation is again turned on, then off, to obtain another burst of urine. This procedure is repeated until the bladder is effectively emptied.
In most prior nerve stimulators the typical shape of the current or voltage pulses that are used are rectangular and monophasic, that is, current or voltage is one direction. The current (or voltage) is applied for a short duration (typically 0.05 to 2 milliseconds) and then the current (or voltage) supply is turned off, and then turned on again in the same direction. This on-off sequence is continued and produces a train of pulses continued, e.g., at a nominal rate of 20 pulses per second, to stimulate the fibers. Thus, the pulse generators produce essentially monophasic unidirectional pulses of current (or voltage).